Light sources such as incandescent lamps, fluorescent lamps, high voltage discharge lamps and the like have been conventionally used for illumination apparatuses. In recent years, light emitting diode (LED) light sources have come into wide use because of their low power consumption and high durability. Since one LED element emits a low light flux, an LED light source uses a plurality of LED elements to obtain substantially the same light flux as conventional light sources. There is a trend to mount LED elements on a substrate as compact as possible to realize a high power LED light source.
An example of such an LED light source may includes a white LED light source which mixes blue light and yellow light to generate white light by coating a resin layer, which contains a fluorescent material to convert the blue light into the yellow light, on a substrate LED element to emit the blue light. There have been proposed various arrangements of fluorescent materials on such an LED substrate from the viewpoint of ease manufacturability and the small amount of fluorescent materials used. Examples of such arrangement of fluorescent materials are shown in FIGS. 6A to 6F.
As one example, as shown in FIGS. 6A and 6B, an LED light source 101 may include a plurality of LED elements 103 mounted on a circular concave portion 104 formed on a substrate 102, and a fluorescent layer 105 formed in the concave portion 104. As another example, as shown in FIGS. 6C and 6D, the LED light source 101 may include a plurality of LED elements 103 and fluorescent layers 105 coated in the form of line on the LED elements 103. As still another example, as shown in FIGS. 6E and 6F, the LED light source 101 may include a plurality of LED elements 103 and fluorescent layers 105 potted on the LED elements 103.
An example of an irradiation pattern of the above-configured LED light source is shown in FIG. 7. In some cases, the irradiation pattern in which light is irradiated to an irradiation surface 106 may show that the color temperature “TA” of light in the central portion of the irradiation surface 106 is higher than the color temperature “TB” of light in the periphery of the irradiation surface 106 and the color temperature decreases from the central portion toward the periphery. The reason for the above will be described below with reference to FIGS. 8A and 8B.
An LED light source 101 shown in FIG. 8A includes a substrate 102, a solid-state light emitting element 103 mounted on the substrate 102, and a fluorescent layer 105 formed on the substrate 102. An LED light source 101 shown in FIG. 8B includes a substrate 102, two adjacent solid-state light emitting elements 103 mounted on the substrate 102, and fluorescent layers 105 potted on the solid-state light emitting elements 103, respectively.
As shown in FIG. 8A, assuming that blue light is emitted from the LED element 103, light emitted perpendicularly to a mounting surface of the substrate 102 is different from light emitted obliquely with respect to the mounting surface in terms of a distance of light propagation through the fluorescent layer 105. That is, a propagation distance “DB” of the obliquely-emitted light is longer than a propagation distance “DA” of the perpendicularly-emitted light and, accordingly, the wave conversion ratio of the obliquely-emitted light irradiated to fluorescent materials is higher than that of the perpendicularly-emitted light.
As a result, the obliquely-emitted light has more wavelength-converted yellow light components than blue light components and accordingly has low color temperature, which may cause the non-uniform color distribution.
Light obliquely emitted from one of the adjacent LED elements 103 as shown in FIG. 8B propagates through a first fluorescent layer 105 covering the corresponding LED element 103 and a second fluorescent layer 105 covering the other LED element 103. This propagation is particularly likely to occur in a light emitting device including a plurality of LED elements arranged with high density. Accordingly, also in the case of the potting, the propagation distance DB of the obliquely-emitted light is longer than the propagation distance DA of the perpendicularly-emitted light, causing the same problem as mentioned above.
In order to prevent non-uniform color distribution, there has been proposed an illumination apparatus including a plurality of LED elements, a fluorescent layer and a V-like groove formed on a surface of the fluorescent layer along a section of the LED elements (see, e.g., Japanese Patent Application Publication No. 2011-60967 (JP2011-060967A). In the illumination apparatus, the propagation distance of obliquely-emitted light is reduced so that the propagation distance of obliquely-emitted light is equal to the propagation distance of perpendicularly-emitted light, as compared to a case where no V-like groove is provided.
However, it may be considered that the LED light source disclosed in JP2011-060967A causes the same problem as in the case of potting since light emitted from one LED element may propagate through the first fluorescent layer covering the corresponding LED element and the second fluorescent layer covering the other LED element.